RFID tag and communication protocol for long range tag communications and power efficiency

ABSTRACT

An RFID tag has a frequency agile RF transceiver that transmits data from the RFID tag to a reader and a direct sequence spread spectrum RF receiver that receives communications from the reader. Messages between the reader and the RFID tag are divided into frames, and each frame contains a frame header transmitted by the reader and at least one time slot containing data transmitted by the RFID tag. The frame header contains a hop sequence and a frequency in a hop sequence to be used by the RFID tag in transmitting data to the reader.

CROSS REFERENECE TO RELATED APPLICATION

This is a Divisional Application of U.S. application Ser. No.10/235,695, now U.S. Pat. No. 7,044,387, filed Sep. 5, 2002, thedisclosure of which is incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a tag that can be suitably attached toan article and that operates in accordance with a communication protocolpromoting long range communications and power efficiency.

BACKGROUND OF THE INVENTION

Various labels have been attached to articles so that the articles canbe distinguished one from the other. For example, bar code labels areattached to articles of grocery and are scanned at a check-out counterin order to automatically identify the articles and to register theprice of the articles as they are purchased.

Bar code labels have also been used in inventory control and monitoring.Accordingly, these bar codes may be scanned in order to track articlesas they move into, through, and out of a storage area. It is also knownto read the bar codes attached to articles in order to access variouscomputer records regarding the articles.

Bar code labels, however, have several drawbacks. For example, computerstored records that are accessed when a bar code is read do not movewith the corresponding article. Therefore, if the article to which thebar code label is attached is remote from the computer, the recordsconcerning that article cannot be immediately accessed if necessary.

Moreover, bar code labels cannot be read remotely. Thus, if it isdesired to take an inventory of articles currently in the storage area,personnel must physically scan each label on each article one at a timein order to determine which articles are presently in the storage area.Such scanning requires the physical presence of the personnel at thelocation of the articles and is extremely time consuming. Additionally,because bar code labels cannot be read remotely, they cannot be used assecurity devices that can be detected if the articles to which they areattached are improperly removed from a secured area.

Instead of bar coded labels, it is known to attach radio frequencyidentification (RFID) tags to the articles to be monitored. The RFIDtags can be read, as can bar code labels. However, unlike bar codelabels, reading RFID tags does not require the physical presence ofpersonnel because the RFID tags can instead be read remotely. Thus,inventory can be taken more quickly because personnel are not requiredto walk around a storage area or other area in order to read the RFIDtags. Moreover, because RFID tags can be read remotely, they can be usedas security devices. Thus, if someone attempts to surreptitiously removean article to which an RFID tag is attached from a secured area, aremote reader can sense the tag and provide an appropriate alarm.

RFID tags can be read one at a time or in groups. When multiple RFIDtags in a group are read at the same time, the information transmittedby the multiple tags frequently collide. Accordingly, spread spectrumtechniques, such as either direct sequence spread spectrum (DSSS) orfrequency hopping, in the communications between the reader and the tagshave been suggested in order to reduce the impact of such collisions. Itis also known to interrogate a tag using either a direct sequence spreadspectrum (DSSS) signal or a frequency hopping signal.

In one embodiment, the present invention combines a direct sequencespread spectrum RF receiver and a frequency hopping transmitter in anRFID tag in order to overcome deficiencies of prior art tags. In anotherembodiment, the present invention relies on a communications protocolthat supports the use of frequency hopping communications between theRFID tag and a reader.

The combination of direct sequence spread spectrum for signal receptionand frequency hopping for signal transmission relative to a tag has notbeen previously suggested. Both of these modulation techniquescircumvent jamming or interference by other signals. Also, the use of adirect sequence spread spectrum RF receiver in a tag permits the tag toproperly synchronize to, and decode, the signal received from the readerin a shorter period of time than if the reader transmits the signalusing frequency hopping. At the same time, this arrangement permits thetag to be interrogated by the reader over long distances whileconserving power.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an RFID tagcomprises a frequency agile RF transmitter and a direct sequence spreadspectrum RF receiver. The frequency agile RF transmitter transmits datato a reader, and the direct sequence spread spectrum RF receiverreceives communications from the reader.

In accordance with another aspect of the present invention, anelectrical signal transmitted between a reader and an RFID tag containsa message divided into frames. Each frame contains a frame headertransmitted by the reader and at least one time slot containing datatransmitted by the RFID tag, and the frame header contains a frequencyin a hop sequence to be used by the RFID tag in transmitting the data.

In accordance with still another aspect of the present invention, amethod of transmitting information contained in a plurality of framesbetween a reader and a tag comprises the following: transmitting a frameheader in each of the frames from the reader to the tag, wherein theframe header contains a frequency in a hop sequence; and, transmittingdata in a time slot of at least one of the frames from the tag to thereader, wherein the data is transmitted at the frequency.

In accordance with yet another aspect of the present invention, a methodof transmitting information contained in a single frame between aplurality of tags and a reader comprises the following: receiving aframe header at each of the tags, wherein the frame header is containedin the frame, wherein the frame also contains a plurality of time slots,and wherein the frame header contains a frequency in a hop sequence andthe number of time slots in the frame; receiving a time slot header ineach of the time slots at each of the tags, wherein each time slotheader contains a number of the corresponding time slot; each of thetags non-deterministically selecting a corresponding time slot; and,each of the tags transmitting data in the time slot that is selected bythe corresponding tag.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will become more apparent from adetailed consideration of the invention when taken in conjunction withthe drawings in which:

FIG. 1 illustrates a tagging system in accordance with one embodiment ofthe present invention;

FIG. 2 illustrates additional detail of a tag that can be used with thetagging system of FIG. 1;

FIG. 3 illustrates additional detail of a reader that can be used withthe tagging system of FIG. 1;

FIG. 4 illustrates a message format useful in supporting communicationsbetween the tag and the reader of FIG. 1;

FIG. 5 illustrates an exemplary composition of a frame of the messageformat shown in FIG. 4;

FIG. 6 illustrates an exemplary composition of the header of the frameshown in FIG. 5;

FIG. 7 illustrates an exemplary composition of a time slot of the frameshown in FIG. 5;

FIG. 8 illustrates an exemplary composition of the header of the timeslot shown in FIG. 6; and,

FIGS. 9 and 10 are flow charts showing an exemplary operation of the tagillustrated in FIGS. 1 and 2.

DETAILED DESCRIPTION

As illustrated in FIG. 1, a tagging system 10 includes a long rangereader 12, a short range reader 14, and an RFID tag 16. The long rangereader 12 includes an antenna 18, and the RFID tag 16 similarly includesan antenna 20. The antennas 18 and 20 establish a long range RF linkbetween the long range reader 12 and the RFID tag 16 so that the longrange reader 12 can remotely read the identification stored in a memoryof the RFID tag 16. The range of the long range reader 12 can be as highas several hundred feet. For example, the long range reader 12 can havean expected range of approximately 500 feet.

A secure link 22 between the short range reader 14 and the RFID tag 16permits the short range reader 14 to read information from the RFID tag16 in a more secure manner. That is, it may not be desirable for thelong range reader 12 to read certain information stored in the RFID tag16 because long range RF communications can be intercepted by astrategically placed surreptitious reader similar to the long rangereader 12. Accordingly, the secure link 22 increases the difficulty inillicitly acquiring the more sensitive information that may be stored onthe RFID tag 16.

The secure link 22 is shown in FIG. 1 as a hard wire link between theshort range reader 14 and the RFID tag 16. Accordingly, the moresensitive information stored on the RFID tag 16 can be read byestablishing a physical interconnection between the short range reader14 and the RFID tag 16. Alternatively, the secure link 22 may be alimited range magnetic link such as those provided by contact-free smartcards. As a still further alternative, the secure link 22 may be a verylimited range RF link. Other alternatives will occur to those skilled inthe art. The expected maximum range of the short range reader 14 overthe secure link 22, for example, may be less than two feet, and isexpected, in typical usage, to be between six inches and eighteeninches.

An embodiment of the RFID tag 16 is shown in additional detail in FIG.2. The RFID tag 16 includes a frequency agile (frequency hopping) RFtransmitter 30 and a direct sequence spread spectrum RF receiver 32. Thefrequency agile RF transmitter 30 and the direct sequence spreadspectrum RF receiver 32 are coupled between the antenna 20 and amicroprocessor 34. Accordingly, the frequency agile RF transmitter 30 ofthe RFID tag 16 implements frequency hopping in transmitting informationto the long range reader 12, and the direct sequence spread spectrum RFreceiver 32 of the RFID tag 16 implements direct sequence spreadspectrum synchronization and decoding in receiving communications fromthe long range reader 12.

The RFID tag 16 also includes an interface 36 between the microprocessor34 and the short range reader 14. Accordingly, the RFID tag 16 cantransmit and receive communications to and from the short range reader14. In the case where the secure link 22 is a hardwire link, theinterface 36 may simply be a plug that is connectible to a correspondingplug of the short range reader 14. In the case where the secure link 22is an RF link, the interface 36 may be an RF transceiver of any knowntype provided that this RF transmitter preferably has a much shorterrange than the frequency agile RF transmitter 30 and the direct sequencespread spectrum RF receiver 32. In the case where the secure link 22 isa magnetic link, the interface 36 may simply be a magneticemitter/sensor capable of magnetically interfacing with the short rangereader 14.

The RFID tag 16 further comprises a memory 38 coupled to themicroprocessor 34. The memory 38 stores the ID of the RFID tag 16 thatcan be read by the long range reader 12 through the antennas 18 and 20,the frequency agile RF transmitter 30, the direct sequence spreadspectrum RF receiver 32, and the microprocessor 34. The memory 38 alsostores information supplied to it by the short range reader 14 throughthe secure link 22, the interface 36, and the microprocessor 34. Thememory 38 can additionally store information supplied by the long rangereader 12.

This information can include, for example, the inventory history of thearticle to which the RFID tag 16 is attached. Accordingly, the date thatthe article entered inventory, the date that the article left inventory,the length of time that the article has been in inventory, any movementinto and out of inventory, and similar information may be stored in thememory 38.

The information stored in the memory 38 may also include shippingmanifests that indicate when and to whom the article is to be shipped.Moreover, in the case where individual articles with differingdestinations are shipped in the same container, an RFID tag attached tothe container, hereafter called a container tag, can be attached to thecontainer. This container tag may be arranged to store the identity anddestination of each article in the container. As articles are removedfrom the container, the information stored in the container tag can beupdated to indicate which articles have been removed, the location atwhich the articles were removed, and the identity of the personnel whoremoved the articles.

The information stored in the memory 36 may further include maintenance,repair, and date of service records showing the maintenance and/orrepair history of the corresponding article.

Other information related to the article may likewise be stored in thememory 38. For example, the integrity of the information stored in thememory 38 can be assured by keeping a record of the modifications to thestored information and of the identity of the personnel making themodifications. As another example, records related to the production ofthe article may be stored in the memory of the tag.

Accordingly, any information about the article may be stored with thearticle instead of in a remote computer system or on paper.

Because the records are carried by the RFID tag 16 attached to acorresponding article, the RFID tag 16 eliminates the need to maintainpaper or computer records of the life history of an article, the RFIDtag 16 eliminates the problem of lost or misplaced records, and the RFIDtag 16 improves operational efficiency by eliminating the requirement toretrieve records prior to accessing and/or operating on the article.

The RFID tag 16 may include a battery (not shown) that is coupled sothat it supplies power to the frequency agile RF transmitter 30, thedirect sequence spread spectrum RF receiver 32, the microprocessor 34,the interface 36 (if necessary), and the memory 38. Moreover, aplurality of sensors (also not shown) may be coupled to themicroprocessor 34. These sensors may include, for example, a temperaturesensor, a humidity sensor, and other sensors such as a pressure sensor,a proximity sensor, an electromagnetic sensor, an optical sensor, amechanical sensor, a chemical sensor, and/or the like. Themicroprocessor 34 stores the information from the sensors in the memory38, and this information may be read from the memory 38 by the shortrange reader 14 or by the long range reader 12.

The microprocessor 34 may be arranged to further sense the voltage levelof the battery. Accordingly, the microprocessor 34 stores this voltagelevel in the memory 38, and this stored voltage level may be read fromthe memory 38 by the short range reader 14 or by the long range reader12. Thus, if the voltage level of the battery as read by either theshort range reader 14 or the long range reader 12 indicates that thebattery needs charging or replacement, suitable remedial action may betaken.

Because of the frequency agile RF transmitter 30 and the direct sequencespread spectrum RF receiver 32, the RFID tag 16 is capable of relativelylong range activation while providing a low power method forcommand-response activation by the long range reader 12. This long rangeactivation allows the RFID tag 16 to be placed at distances remote fromthe long range reader 12 for purposes of interrogating the RFID tag 16for its unique tag number and possibly other information.

The frequency agile RF transmitter 30 and the direct sequence spreadspectrum RF receiver 32 allow the tagging system 10 to operate in theFCC defined Industrial Scientific and Medical (ISM) bands at maximumlegal power. Both frequency hopping as used by the frequency agile RFtransmitter 30 and direct sequence spread spectrum communications asused by the direct sequence spread spectrum RF receiver 32 circumventjamming by narrow-band signals using different methods of spreading thesignal over a large bandwidth. The direct sequence spread spectrum RFreceiver 32 can receive signals from the long range reader 12 withinmilliseconds of activation. By contrast, a frequency agile receiver mustsearch a long frequency hopping sequence in order to receive signalsfrom the long range reader 12. The time required to make this search istypically longer than the time required to detect a direct spreadspectrum sequence because the direct spread spectrum signal is either ona fixed frequency or on one of only a few frequencies.

An embodiment of the long range reader 12 is shown in additional detailin FIG. 3. The long range reader 12 includes a direct sequence spreadspectrum RF transmitter 50 and a frequency agile RF receiver 52 coupledbetween the antenna 18 and a microprocessor 54. The frequency agile RFreceiver 52 of the long range reader 12 implements frequency hopping inreceiving information from the frequency agile RF transmitter 30 of theRFID tag 16. Moreover, the direct sequence spread spectrum transmitter52 of the long range reader 12 implements direct sequence spreadspectrum transmission in transmitting communications to the directsequence spread spectrum RF receiver 32 of the RFID tag 16.

The long range reader 12 further comprises a memory 56 coupled to themicroprocessor 34. The memory 56 stores the information that the longrange reader 12 receives from the RFID tag 16. The memory 56 also storesthe software that supports the communication protocol as describedherein.

This communication protocol governs the message format that is usedbetween the long range reader 12 and the RFID tag 16. According to thisprotocol, a message is comprised of a plurality of frames as shown inFIG. 4. Each frame is preferably no longer than the length of time thefrequency agile RF transmitter 30 is allowed to dwell at any givenfrequency.

Each of the frames shown in FIG. 4 has the construction shown in FIG. 5.Accordingly, each frame has a frame header and a number of time slotsTS0-TSN. The frame header contains information about the long rangereader 12 that is reading the RFID tag 16. As shown in FIG. 6, the frameheader contains (i) the state of the long range reader 12, (ii) the hopsequence currently being used by the long range reader 12 to receivemessages from the RFID tag 16, (iii) and the current position (i.e.,frequency) of the long range reader 12 in this hop sequence. The frameheader can also contain such other information that is useful in thetagging system 10. For example, the frame header may also contain thenumber (N+1) of time slots in the corresponding frame.

The long range reader 12 may have several reader states including, forexample, an active communication state and a beacon state. In the activecommunication state, the long range reader 12 commands responses fromone or more selected tags such as the RFID tag 16. In the beacon state,the tags, such as the RFID tag 16, self-initiate the transmission ofmessages to the long range reader 12.

The hop sequence and/or the current position in the hop sequence ascontained in the frame header are/is useful to tags that have limitedsignal processing capability. Such tags for example, may have nocapability themselves to determine the frequency (i.e., the currentposition in the hop sequence) onto which they should transmit theirresponses.

Moreover, each time slot may also include a time slot header and data asshown in FIG. 7, and each time slot header, as shown in FIG. 8, maycontain the hop sequence and the current position in the hop sequence ofthe long range reader 12. The time slot header may also contain therelative position, such as a time slot number (0, 1, . . . , or N), ofthe corresponding time slot in the frame. This relative positioninformation may be used by the RFID tag 16 to establish a relativetiming interval into which the RFID tag 16 can transmit data. Bytransmitting the hop sequence and the current position in the hopsequence at the beginning of each time slot, the RFID tag 16 is aided inits rapid acquisition of the current hop sequence and frequency. Becausethe RFID tag 16 can acquire, from the time slot header in each timeslot, sufficient information about the frequency and timing of the longrange reader 16, the RFID tag 16 may power down until such time that itexpects the complete header information to be transmitted by the longrange reader 12. Therefore, the RFID tag 16 is able to substantiallyreduce the amount of power that it uses to determine the frequency andtiming to be used by its frequency agile RF transmitter 30 intransmitting information in the data portion of the time slot.

As indicated above, the long range reader 12 transmits all headers,whether frame headers or time slot headers. The RFID tag 16 transitsonly in the data portion of the time slots. The RFID tag 16 mayimplement a non-deterministic method of selecting a time slot for thetransmission of data. By using a non-deterministic method of selecting atime slot, the possibility of a plurality of tags transmitting data intothe same time slot is minimized. For purposes of illustration, such anon-deterministic method of selecting a time slot could be embodied by apseudo-random number generator that pseudo-randomly generates a numberof a time slot into which its corresponding tag transmits its data. Thisimplementation results in a communications protocol similar to, but notidentical to, the Aloha protocol, a standard communications protocol.

The long range reader 12 can communicate directly with a specific tag ora group of specific tags. When the long range reader 12 is communicatingdirectly with a specific tag or a group of specific tags, the long rangereader 12 may suspend the transmission of time slot headers. Thissuspension indicates to all other tags that their communications are tobe suspended. Also, all data may be transmitted between the long rangereader 12 and the RFID tag 16 in packets having packet numbers so thatboth the long range reader 12 and the RFID tag 16 can detect missing orduplicate data. Moreover, acknowledgements can be used to signify asuccessful transmission between the long range reader 12 and the RFIDtag 16. A failure to receive an acknowledgement can causere-transmission of the information. Once a transaction between the longrange reader 12 and a specific tag or group of tags is complete, thelong range reader 12 resumes transmitting the headers.

As shown in FIG. 9, when the RFID tag 16 detects a received message atthe direct sequence spread spectrum RF receiver 32 as indicated by ablock 100, the RFID tag 16 parses the header information as indicated bya block 102 and, as indicated by a block 104, stores the reader state,the hop sequence, the current position in the hop sequence, and anyother information that is contained in the header.

As shown in FIG. 10, when it is time for the RFID tag 16 to transmitdata as indicated by a block 106, the RFID tag 16 selects the time slotin which it is to transmit the data as indicated by a block 108, theRFID tag 16 selects the frequency at in which it is to transmit the dataas indicated by a block 110, and the RFID tag 16 causes the frequencyagile RF transmitter 30 to transmit the data in the selected time slotusing the selected frequency as indicated by a block 112.

The time at which the RFID tag 16 is to transmit data (the block 106)depends on the state of the long range reader 12. If the reader state ascontained in the header and stored by the RFID tag 16 (block 104)indicates that the long range reader 12 is in the beacon mode, the RFIDtag 16 self-originates the transmission of data. In this state, the RFIDtag 16, for example, may be arranged to transmit data periodically basedon a timer. When the timer indicates that it is time to transmit, theRFID tag 16 pseudo-randomly selects a time slot as indicated by theblock 108, selects a transmission frequency as indicated by the block110, and transmits as indicated by the block 112.

If the long range reader 12 is in an active communication state, theRFID tag 16 determines that it is time to transmit when it receives aninterrogation message from the long range reader 12. When the RFID tag16 receives an interrogation message, the RFID tag 16 pseudo-randomlyselects a time slot as indicated by the block 108, selects atransmission frequency as indicated by the block 110, and transmits asindicated by the block 112.

Other reader states for the long range reader 12 are also possible.

As indicated above, the time slot in which the RFID tag 16 transmitsdata may be selected based on the pseudo-randomly generated number. Thefrequency at which the RFID tag 16 transmits data is selected based onthe current position in the hop sequence as received (block 100) andstored (block 104) by the RFID tag 16 (block 100).

Certain modifications of the present invention have been disclosedabove. Other modifications will occur to those practicing in the art ofthe present invention. For example, the functions of the long rangereader 12 as described above have been confined to reading informationfrom the RFID tag 16. However, the long range reader 12 can also bearranged to write information to the RFID tag 16.

Also, as described above, the long range reader 12 is arranged to readthe tag ID of the RFID tag 16, and the short range reader 14 is arrangedto read other information from the RFID tag 16. However, the long rangereader 12 may be arranged instead to read any combination of tag ID andother information from the RFID tag 16, and the short range reader 14may be similarly arranged to read any combination of the tag ID andother information from the RFID tag 16.

Moreover, although the RFID tag 16 is shown as a microprocessor basedtag in FIG. 2, the RFID tag 16 may instead comprise one or more digitalcircuit elements, and/or a programmable logic array, and/or a dedicatedintegrated circuit, etc.

Furthermore, the long range reader 12 as described above has a range ofseveral hundred feet and could have an expected range of approximately500 feet. However, this range could be longer or shorter depending onthe application and/or other factors. Similarly, the range given abovefor the short range reader 14 could be other than as described above.

Accordingly, the description of the present invention is to be construedas illustrative only and is for the purpose of teaching those skilled inthe art the best mode of carrying out the invention. The details may bevaried substantially without departing from the spirit of the invention,and the exclusive use of all modifications which are within the scope ofthe appended claims is reserved.

1. An electrical signal transmitted between a reader and RFID tags,wherein the electrical signal contains a message divided into frames,wherein each frame contains a frame header transmitted by the reader anda plurality of time slots, wherein at least one of the time slotscontains data transmitted by a corresponding RFID tag, wherein each ofthe time slots in a frame comprises a slot header, wherein each of theslot headers contains a frequency in a hop sequence to be used by atleast one of the RFID tags in transmitting data, and wherein the frameheader contains a frequency in a hop sequence to be used by at least oneof the RFID tags in transmitting data.
 2. The electrical signal of claim1 wherein the frame header also contains the hop sequence.
 3. Theelectrical signal of claim 1 wherein the frame header also contains areader state.
 4. The electrical signal of claim 3 wherein the readerstate indicates that the RFID tag is to operate in a beacon mode.
 5. Theelectrical signal of claim 3 wherein the reader state indicates that theRFID tag is to operate in an active communication mode.
 6. Theelectrical signal of claim 5 wherein the reader state indicates that theRFID tag is to operate in a beacon mode.
 7. The electrical signal ofclaim 3 wherein the frame header further contains the hop sequence. 8.The electrical signal of claim 7 wherein the reader state indicates thatthe RFID tag is to operate in a beacon mode.
 9. The electrical signal ofclaim 7 wherein the reader state indicates that the RFID tag is tooperate in an active communication mode.
 10. The electrical signal ofclaim 9 wherein the reader state indicates that the RFID tag is tooperate in a beacon mode.
 11. The electrical signal of claim 1 whereineach of the time slots contains a data portion for the transmission ofdata by a corresponding RFID tag, and wherein the data portion of atleast one of the time slots contains the data transmitted by acorresponding RFID tag at the frequency contained in the slot header.12. The electrical signal of claim 11 wherein the frame header and thetime slot headers also contain the hop sequence.
 13. The electricalsignal of claim 11 wherein the frame header also contains a readerstate.
 14. The electrical signal of claim 13 wherein the reader stateindicates that the RFID tag is to operate in a beacon mode.
 15. Theelectrical signal of claim 13 wherein the reader state indicates thatthe RFID tag is to operate in an active communication mode.
 16. Theelectrical signal of claim 15 wherein the reader state indicates thatthe RFID tag is to operate in a beacon mode.
 17. The electrical signalof claim 13 wherein the frame header and the time slot headers furthercontain the hop sequence.
 18. The electrical signal of claim 17 whereinthe reader state indicates that the RFID tag is to operate in a beaconmode.
 19. The electrical signal of claim 17 wherein the reader stateindicates that the RFID tag is to operate in an active communicationmode.
 20. The electrical signal of claim 19 wherein the reader stateindicates that the RFID tag is to operate in a beacon mode.
 21. Theelectrical signal of claim 1 wherein at least one of the time slotscontains data transmitted by the reader.
 22. A method of transmittinginformation contained in a plurality of frames between a reader and aplurality of tags comprising: transmitting a frame header in each of theframes from the reader to the tags; and transmitting a slot header ineach of the time slots from the reader to the tags, wherein each of theslot headers contains a frequency in a hop sequence to be used intransmitting data from the tags to the reader.
 23. The method of claim22 wherein the frame header also contains the hop sequence.
 24. Themethod of claim 22 wherein the frame header also contains a readerstate.
 25. The method of claim 24 wherein the reader state indicatesthat the RFID tag is to operate in a beacon mode.
 26. The method ofclaim 24 wherein the reader state indicates that the RFID tag is tooperate in an active communication mode.
 27. The method of claim 26wherein the reader state indicates that the RFID tag is to operate in abeacon mode.
 28. The method of claim 24 wherein the frame header furthercontains the hop sequence.
 29. The method of claim 22 wherein each ofthe time slots contains a data portion transmitted by a correspondingRFID tag, and wherein the data portion of at least one of the time slotscontains the data transmitted by a corresponding RFID tag at thefrequency contained in the slot header.